This invention relates to the field of hollow fiber production, especially to fluoropolymer-coated polyolefin hollow fibers for use in fluid separation membranes and a process for making same.
In creating a gas or liquid separation membrane, it is desirable to have both a high rate of permeation or throughput and a high separation factor. This combination of characteristics permits the effective separation of a relatively large volume of fluid per unit time. Large rates of permeation also allow the membrane to be operated at lower pressures, which improves cost-effectiveness.
Highly porous membranes tend to be very permeable, but do a poor job of separating fluids into their components. Less porous, dense membranes can be more selective, but at the cost of reduced throughput.
Asymmetric membranes generally have a thin, dense separation layer and a less dense, more permeable layer. These asymmetric membranes are often chosen for separation applications because they may provide a good combination of permeation and separation. Hollow fibers having a dense skin and a porous body can be used to make such membranes.
Unfortunately, many of the materials, e.g. fluoropolymers, used to make hollow fluid separation fibers is relatively expensive. Some processes are also expensive, particularly if many process steps are needed. Furthermore, in some fibers the permeability, the separation factor, or both may be rather low.
U.S. Pat. No. 4,230,463 issued to Henis, et al. describes multi-component membranes for gas separations wherein a porous membrane is coated with another substance. The coating is said to enhance the separation capability of the membrane.
U.S. Pat. No. 4,290,987 issued to Soehngen, et al., describes a process for making microporous hollow fibers comprising a polyolefin by solvent stretching a nonporous hollow precursor fiber having a controlled degree of molecular orientation.
U.S. Pat. No. 4,871,494 issued to Kesting, et al. describes a process for forming asymmetric gas separation membranes having graded density skins.
U.S. Pat. No. 4,756,932 issued to Puri describes a process for making highly permeable coated composite hollow fiber membranes. The hollow fiber substrate is passed continuously through a polymeric coating solution and withdrawn through a coating die to form a uniform coating.
U.S. Pat. No. 4,881,954 issued to Bikson, et al. describes permeable membranes for enhanced gas separation in which an asymmetric porous support is coated with a separation layer. The support is made asymmetric prior to coating to provide mechanical stability and a uniform support surface.
U.S. patent application Ser. No. 686,739 filed on Apr. 17, 1991 by Chung, et al. describes a method for making asymmetric hollow fibers from fluoropolymers. The outer layer of these fibers is coated with a highly permeable substance to achieve high selectivities.
U.S. patent application Ser. No. 738,996 filed on Aug. 1, 1991 by Chung, et al. describes an asymmetric hollow fiber consisting essentially of SIXEF.TM.-Durene, a polyimide made from 2,2-bis[3,4-dicarboxyphenyl] hexafluoropropane dianhydride and 2,3,5,6-tetramethylphenylene diamine monomers.
U.S. patent application Ser. No. 739,063 filed on Aug. 1, 1991 by Chung, et al. describes an asymmetric hollow fiber made from SIXEF.TM.-Durene coated with poly-4-vinylpyridine ("PVP").
The disclosures of the foregoing references are incorporated by reference. Although they are not necessarily prior art, these references do reflect the present state of the art.